Sedentariness is common and getting worse. It is a serious risk factor for many diseases. I am often surprised by patients who think they are physically active who are in fact quite sedentary. This post is meant to review what counts as being sedentary and what counts as being physically inactive. These definitions draw in part on a proposal from the recommendations of The Sedentary Behaviour Research Network 2012.
If you are awake, mostly seated or lying down, and expending less than or equal to 1.5 metabolic equivalents (MET), then you are sedentary.
One MET is about how much energy someone is burning at complete rest while seated or lying down, such as while watching television. This rises to 1.5–1.8 when one is sitting and typing or playing a video game, and rarely really gets one out of the sedentary range.
Most people spend several hours a day completely sedentary in front of a computer/video game console, at a desk, or in school.
Some other pragmatic measures of sedentariness: If you walk less than 3 miles a day (about 2,000 steps per mile for the average person, so 6,000 steps a day or less), sit more than 12 hours a day, or engage in no exercise that causes your heart rate or breathing rate to go up.
Technically, failure to engage in moderate-to-vigorous exercise would count as inactivity.
Some other MET values for exercises can be found at Wikipedia.
References
Sedentary Behaviour Research Network. Letter to the Editor: Standardized use of the terms “sedentary” and “sedentary behaviours.” Appl Physiol Nutr Metab 2012;37(3):540-542.